1. Field of the Invention
This invention relates to IC amplifiers for low-input current applications. More particularly, this invention relates to such amplifiers employing low-leakage-current Junction Field Effect Transistors (JFETs) having electrically-isolated top and back gates.
2. Description of the Prior Art
Monolithic amplifiers for low input current applications generally have used MOSFET or JFET input stages. MOSFET devices have very low leakage, but have disadvantages. For example, they require over-voltage protection diodes which can increase the net input current to 1 pA or more. Also, the low-frequency voltage resolution of MOSFET inputs is limited by high 1/f noise due to surface effects on the MOSFET channel.
Ion-implanted JFETs are available with an n+ shield connected to the pocket n-epitaxy of the device to form a gate on each side of the p+ implanted channel. This greatly reduces voltage noise compared to MOSFETs. However, leakage current from the back gate to the substrate and the isolation junctions raises the input current substantially, e.g. to 2 pA. Dielectric isolation has been used to eliminate back gate to substrate leakage, making possible amplifiers with gate currents of 1 pA.
Recently, JFETs have been developed with electrically-isolated top and back gates. One particularly advantageous arrangement of that kind is described in U.S. patent application Ser. No. 658,270 (J. F. Lapham et al.), filed on Oct. 5, 1984. The JFET shown there avoids the usual conductive connection between the top and back gates by the use of closed geometry for the source/drain regions. That is, one source/drain region fully surrounds (in a lateral sense) the other source/drain region. A non-penetrating contact layer is provided on the upper surfaces of the top gate, to prevent the material of the contact layer from entering the top gate region to any significant extent, thereby avoiding possible conductive connection through to the back gate by that route. With the top and back gates thus isolated electrically, the JFET becomes a four-terminal device, with independent access available to all four terminals.
Although such JFETs have extremely low leakage current into their top gates, e.g. 50 fempto-amps, there has been a problem in providing suitable amplifier circuitry which is capable of taking full advantage of the special low-leakage-current chracteristics of four-terminal JFETs.